“Dry contacts” include contacts of a type of switch which does not carry power on a normal basis, i.e. not intended to carry power as a part of an operational circuit. Dry contact switches are used in a variety of applications as an input method to an electronic device. For example, dry contacts are used to select settings on switches such as dual-in-line packages (DIP) switches for an electronic device. Dry contact switches are polled with a wetting current to determine their state (e.g., ON or CLOSED and OFF or OPEN) in order to perform certain operations. The wetting current is a current that is used to clean surface oxidation, if present, on the dry contacts of the switch. A wetting current parameter defines a minimum current, for the wetting current, that is necessary for cleaning surface oxidation on the dry contacts of the switch to be properly conductive.
Circuits, such as current sources and multiplexors, are used to supply wetting current for dry contacts, but are costly and consume a significant amount of power. Furthermore, it may not be desirable or even possible to supply wetting current to the dry contacts of a switch directly from the power supply of a low-powered and/or self-powered electronic device. A low-powered electronic device includes devices that draw around tenths of a watt or less of power. A self-powered electronic device includes devices that have a separate power supply which may be used in case of power failure or shut down of the main lines as part of the protective function of the electronic device, to maintain the protective capability of the electronic device in the absence of line power. These types of electronic devices may need to limit the average current consumed when reading or sensing the state of the switch. The terms “read” and “sense” and their derivatives are used interchangeably herein.
For example, a self-powered electronic device, such as a self-powered circuit breaker or motor overload relay, may employ a current transformer (CT) to supply operating current (also known as “supply current”) to the device inductively as well as to produce a measurement signal used for polling an operational parameter of the device which is set by the position of the dry contact switch. The electronic device may perform one or more functions, such as load protection (e.g., overcurrent or overload protection), based on the operational parameter. However, measurement error of the measurement signal can be a non-linear function of the power drawn by the electronic device and its components. Thus, if the electronic device draws too much power to produce the wetting current, it may increase the possibility of an erroneous measurement signal, which in turn impacts the functions of the electronic device that depend on the measurement signal.
Accordingly, there is a need to provide a simple and cost effective wetting current circuit for an electronic device. There is also a need to provide a wetting current circuit that limits or reduces power consumption from a power supply of the electronic device.